The present invention relates to a method according to the preamble of claim 1 and to a device according to the preamble of claim 3, and concerns in particular such forms of electric drives which enable high reduction ratios to be achieved at moderate technical input in the winding area.
It has been known before in connection with induction motors or asynchronous motors to arrange the stator winding in grooves inside the stator cavity in such a distribution that the windings of the individual phases follow each other in correspondence with the displacement in time of the voltages and currents, i.e. at 120.degree., electrically, in the case of a three-phase a.c. motor.
In the case of multi-phase asynchronous motors, therefore, the time sequence of the peak values of the stator current of the individual phases leads to a magnetic rotary field which, according to the known formula ##EQU1## leads to n revolutions per minute for a given frequency (f) and a given number of pole pairs (p). The rotor, for example a squirrel cage rotor, tends to follow this rotary field, but does never fully reach the latter's speed. This behavior or the rotor, which is insofar asynchronous, being affected by a certain slip, causes a voltage to be induced in the rotor by the overrunning field, which voltage rises as the slip rate increases. Consequently, the rotor current rises, for a given rotor resistance, in proportion to the slip rate which, as in the case of a transformer, is covered from the primary side, i.e. from the stator, through the action of the magnetic field.
Now, it is general usage in certain applications--an example to be mentioned here is the operation of washing machines, although it is understood that the invention is not restricted to such an application--to make use or single-phase induction motors with two speeds which are usually supplied with current from the likewise single-phase mains available practically everywhere. As the washing machine, which has been mentioned by way of example, is normally operated at two, sometimes very different speeds, namely a highest possible tumbling speed and, on the other hand, the slow washing speeds, which are moreover interrupted by reversing movements, the only single-phase induction motor used in this case is designed in such a way, for example by convenient distribution and assignment of the coil pairs, as to form, for example, a two-pole partial induction motor and a 16-pole partial induction motor so that--as will be explained in more detail further below--speeds of, for example, 3000 min.sup.-1 and 375 min.sup.-1, respectively, are obtained, to provide some numerical figures for the better understanding.
It has been found, however, that the desired high speed reduction ratio is very difficult to achieve, even for such combined single-phase induction motors supplied from single-phase mains, and that in addition the cost factor is a problematic aspect, due to the high number of winding coils required.
Now, it is the object of the present invention to provide a method and a device for the operation of electric induction motors which ensure that at least the same, or even higher speed reduction ratios can be achieved, with a reduced input and a lesser number of coil pairs, and at altogether lower cost.